AML1-ETO is a DNA binding fusion protein generated from t(8;21). This chromosomal translocation is one of the most common genetic abnormalities in acute myeloid leukemia (AML), identified in over 10% of all cases. Studies from our own work and from others have demonstrated that AML1-ETO plays an important role in the development of leukemia. However, it is not sufficient by itself for leukemia development. Interestingly, multiple forms of AML1-ETO are present in t(8; 21) leukemia samples due to alternative RNA splicing and deletion mutations. However, most of the current studies are focused on the function of the full length AML1-ETO in leukemogenesis. The role played by these other shorter forms have been largely ignored. Recently, we performed bone marrow transplantations on mice transduced with a full length AML1-ETO expression vector. Three of these mice developed AML, two of which contained C-terminal truncations as the only form of AML1-ETO present in the tumor. Further mouse model analysis revealed that the C-terminal truncated AML1-ETO was a potent inducer of leukemia. These findings lead to the hypothesis that in addition to the already described interactions between AML1-ETO and the histone deacetylase complexes, alterations of protein - protein interactions in the presence of the shorter forms of AML1-ETO or additional genetic mutations which disrupt protein interactions with the C-terminal portion of full length AML1-ETO may contribute to t (8; 21) involved leukemogenesis. We have developed three specific aims to test this hypothesis. Specific Aim #1 will analyze the effect of various forms of AML1-ETO on leukemogenesis and hematopoiesis using mouse models. Specific Aim #2 will study the effect of various forms of AML1-ETO on cell survival, proliferation, and differentiation using a well-established multipotent hematopoietic cell line. Specific Aim #3 will characterize proteins interacting with AML-ETO to understand the molecular mechanism of AML1-ETO involved leukemia development. We have established animal and cell line models and biochemical approaches to pursue these proposed studies. The experiments may provide valuable insight into the molecular mechanisms of leukemogenesis and therapeutic drug designs in cancer treatment.